Inner Flame Front Structures and Burning Velocities of Premixed Turbulent Planar Ammonia/Air and Methane/Air Flames

Abstract

Ammonia ($$\mathrm {NH_3}$$ NH 3 ) has attracted interest as a future carbon-free synthetic fuel due to its economic storage and transportation. In this study, quasi direct numerical simulations (quasi-DNS) with detailed-chemistry have been performed in 3-D to examine the flame thickness and assess the validity of Damköhler’s first hypothesis for premixed turbulent planar ammonia/air and methane/air flames under different turbulence levels. The Karlovitz number is systematically changed from 4.26 to 12.06 indicating that all the test conditions are located within the thin reaction zones combustion regime. Results indicate that the ensemble average values of the preheat zone thickness deviate slightly from the thin laminar flamelet assumption, while the reaction zone regions remain relatively intact. Following the balance equation of reaction progress variable gradient, normal strain rate and the tangential diffusion component of flame displacement speed variation in the normal direction to the flame surface are found to be responsible for thickening the flame. However, the sum of reaction and normal diffusion components of flame displacement speed variation in the normal direction to the flame surface is in charge of flame thinning for ammonia/air and methane/air flames. In addition, the validity of Damköhler’s first hypothesis is confirmed by indicating that the ratio of the turbulent burning velocity to the unstrained premixed laminar burning velocity is relatively equal to the ratio of the wrinkled to the unwrinkled flame surface area. Furthermore, the probability density functions of the density-weighted flame displacement speed show that the bulk of flame elements propagate identical to the unstrained premixed laminar flame.

A study of flame dynamics and structure in premixed turbulent planar NH3/H2/air flames

“…This model was previously used for studying spray-assisted dual-fuel ignition, 46,47 spray-assisted tri-fuel ignition, 48 and premixed turbulent flame propagation. 49…”

“…This model was previously used for studying spray-assisted dual-fuel ignition, 46,47 spray-assisted tri-fuel ignition, 48 and premixed turbulent flame propagation. 49 Inflow and outflow boundary conditions with a zero mean inlet velocity in the direction of the mean flame propagation are used. In the lateral direction, the periodic boundary conditions are employed, indicating an unbounded planar flame brush.…”

Editorial –‘Thermo- and fluid-dynamics of clean propulsion powerplants’: THIESEL 2022 Special Issue by Xandra Margot

“…Indeed, five out of the nine papers presented here, deal with studies on combustion with diverse types of fuels, including ammonia and hydrogen. 5–9 Closely linked to this topic is naturally the fuel injection process since this is paramount to achieve a controlled and efficient combustion. 10 This has been one of the important topics of the conference too and three papers on this have been selected for this special issue.…”

“…As stated above, this special issue includes two papers about the use of ammonia as eco-fuel, 5,8 one on dual-fuel combustion for heavy-duty applications, 6 two on investigation related to hydrogen engines, 7,9 two on ducted fuel injection, 11,12 plus one on needle and armature dynamics of direct injectors. 13 Finally, the last one is on the fuel consumption of mobile air-conditioning systems in electrified vehicles.…”

“…Pelé et al 8 investigated experimentally combustion and emissions characteristics of a spark-ignition engine resulting from the injection of pure ammonia and of different ethanol-ammonia blends, with different strategies to reach homogeneous or stratified conditions at various intake pressures. Tamadonfar et al 5 on the other hand performed some quasi-DNS simulations to study the flame dynamics and structure of stoichiometric planar ammonia /hydrogen/air flames under different turbulent conditions. Longo et al 6 have addressed the impact on heavy-duty engines emissions associated to the use of Diesel-hythane dual-fuels.…”